This invention relates to a process for preparing (meth)acrylic acid. In particular, the invention relates to a process for preparing (meth)acrylic acid which utilizes an aqueous stream which includes recycled wastewater in the process to separate product (meth)acrylic acid from a mixed product gas.
Acrylic acid is generally prepared by the catalytic oxidation of at least one hydrocarbon material. For instance, acrylic acid may be prepared from propylene and/or acrolein in a one or two step process. In a first step propylene is oxidized in the presence of oxygen, diluent inert gases, water vapor, and appropriate catalysts to produce acrolein according to equation (I):
C3H6+O2C2H3CHO+H2O+heat (I).
The acrolein is then oxidized, in a second step, in the presence of oxygen, diluent inert gases, water vapor, and appropriate catalysts to form acrylic acid according to equation (II):
C2H3CHO+xc2xdO2C2H3COOH+heat (II).
The acrolein may be provided as starting material in a one step reaction (II) to produce acrylic acid.
Alternatively, propane may be used as a starting material. The propane is oxidized using appropriate catalysts, for instance, as described in U.S. Pat. No. 5,380,933 to form product acrylic acid.
Methacrylic acid is similarly prepared by catalytic oxidation of isobutylene and/or isobutane.
The acrylic acid prepared using such catalytic oxidation reactions is present in a mixed product gas exiting the reactor. Generally, the mixed product gas is cooled and is contacted with an aqueous stream in an absorption tower, thereby providing an aqueous acrylic acid solution which is then dehydrated in a distillation step to provide a crude acrylic acid solution. The crude acrylic acid solution can be used to produce various acrylic esters or be further purified to provide various grades of purified acrylic acid which can then be further utilized, for instance in the production of super absorbent polymer products.
Typically in chemical manufacturing processes, including such processes for the production of acrylic acid, a large waste load is generated. Such waste load usually takes the form of waste product gases and waste water streams. Waste product gases may be generated at several points in the acrylic acid manufacturing process. Of particular interest is the remainder of the mixed product gas emerging from the absorber which has had product acrylic acid absorbed from it upon contact with the aqueous stream in the absorber. This remainder of the mixed product gas, known as the absorber waste gas or absorber off-gas, typically undergoes some sort of waste treatment such as thermal oxidation, incineration, or catalytic oxidation before being released to the air so as to comply with applicable environmental requirements.
Wastewater is also generated in the acrylic acid production process as well as in other manufacturing processes. A typical acrylic acid production facility may generate up to two pounds of wastewater per pound of acrylic acid produced, depending on the particular process used. Of particular interest is the wastewater recovered in dehydration processes, such as in the dehydration of acrylic acid. Generally such wastewater also must be treated in some manner to comply with applicable environmental standards before being released into the environment. Consequently, since treatment and disposal of waste gases and wastewater constitutes a significant expense for the acrylic acid manufacturer alternative uses and treatments which add value to the manufacturing process and/or reduce expenses is a constant goal to such manufacturers.
Several methods have been developed in the art to meet this goal. It is known in the art to recycle at least a portion of the absorber off-gas back to the reactor(s). This recycle serves several purposes including providing inert diluent gas and steam to the reactant composition, and reducing wastewater generated by the process by reducing the amount of steam that is fed to the process. Furthermore, small amounts of unreacted propylene and acrolein contained in the off-gas are given another chance to react and thereby improve the overall acrylic acid yield by increasing conversions of propylene and acrolein.
Recycle of certain generated wastes in an acrylic acid process for reuse in the process absorbing step has been taught in the art. For instance, Japanese Patent Application Kokai (Laid Open) No. 246941/1993 teaches recycle of a recovered acetic acid solution for reuse as an absorbent in an acrylic acid absorption tower. However, the application states that acrylic acid recovery is inefficient in the absorber because no solvent and substantially no acrylic acid is present. U.S. Pat. No. 5,785,821 discloses wastewater recycle to the absorber of an acrylic acid process wherein the recycled wastewater stream has a specific composition of acetic acid (3-10 wt %), acrylic acid (0.5-5.0 wt %), and distillation solvent (0.01-0.5 wt %). Such a recycle stream, containing these specific amounts of acetic acid, acrylic acid and distillation solvent is said to enable collection of acrylic acid in the absorber at a high efficiency.
However, one problem envisioned with recycle of wastewater is that the recycled wastewater may contain an unacceptable concentration of undesirable components such as volatile organic compounds (VOC). Such high concentrations of undesirable components in the recycled wastewater stream can lead to a higher waste concentration, e.g., acetic acid and organic distillation solvent, in the absorber off-gas. Consequently, when some or all of an absorber off-gas is recycled to the reactor it will contain a higher level of undesirable components. Such materials may be detrimental to the catalytic oxidation reactions which form acrolein and/or acrylic acid in the reactor. In particular, it is believed that activity of the oxidation catalysts may be reduced by the presence of the higher level of volatile organic components. Furthermore, certain organic, such as toluene, may actually compete with starting materials so that less starting materials are reacted and more by-products show up in the acrylic acid product.
The preparation and isolation of methacrylic acid proceeds by similar steps. Consequently, methacrylic acid manufacturers suffer from similar problems.
The present inventors have now discovered that by stripping the wastewater of undesirable components using a stripping gas including process waste gas overcomes the problem of unsuitable levels of undesirable components being recycled to the absorber. Furthermore, the invention enables obtaining additional value from process wastewater and waste gas streams. This is done while maintaining a suitable acrylic acid collection efficiency in the absorber.
Accordingly, a novel process for preparing acrylic acid is described herein wherein the following advantages are provided:
(1) a decreased amount of undesirable components in the absorber off-gas leading to a decreased amount of potentially harmful components recycled back to the reactor;
(2) additional value is obtained from process waste gas streams by using them to strip process wastewater streams;
(3) stripped wastewater having a lower level of components potentially harmful to the catalytic reactions in the reactor are recycled to the absorber for acrylic acid absorption thereby reducing the wastewater load in the facility;
(4) stripping gas streams sent to thermal oxidizers for treatment have a higher concentration of organic material from the stripping process, thereby reducing the fuel requirement of the thermal oxidizer.
In one aspect of the present invention, there is provided a process for preparing (meth)acrylic acid, including the steps of: (A) feeding to an absorption tower (i) a mixed product gas from the catalytic oxidation of at least one hydrocarbon material with a molecular oxygen containing gas, and (ii) an aqueous stream including recycled wastewater and less than 3.0 percent by weight acetic acid; (B) contacting the mixed product gas with the aqueous stream in the absorption tower to form an aqueous (meth)acrylic acid stream; and (C) feeding the aqueous (meth)acrylic acid stream to a distillation column, wherein it is subjected to azetropic distillation in the presence of at least one distillation solvent to form a (meth)acrylic acid solution substantially free of water.
In a second aspect of the present invention, there is provided a process for preparing (meth)acrylic acid, including the steps of (A) feeding to an absorption tower a mixed product gas from the catalytic oxidation of at least one hydrocarbon material with a molecular oxygen containing gas, and an aqueous stream comprising recycled wastewater and less than 3.0 percent by weight acetic acid, wherein prior to being added to the aqueous stream at least a portion of the recycled wastewater is stripped of undesirable components in a stripping column using a stripping gas which includes a waste gas stream; (B) contacting the mixed product gas with the aqueous stream in the absorption tower to form an aqueous (meth)acrylic acid stream; (C) feeding the aqueous (meth)acrylic acid stream to a distillation column, wherein it is subjected to azeotropic distillation in the presence of at least one distillation solvent to form a (meth)acrylic acid solution substantially free of water.
In a third aspect of the present invention, there is provided a process for preparing (meth)acrylic acid, including the steps of: (A) stripping at least a portion of a recycled wastewater stream in a stripping column, using a stripping gas which includes an absorber off-gas, to form an aqueous stream having less than 3.0 percent by weight acetic acid; (B) feeding (i) a mixed product gas from the catalytic oxidation of at least one hydrocarbon material with a molecular oxygen containing gas, and (ii) the aqueous stream to an absorption tower; (C) contacting the mixed product gas with the aqueous stream in the absorption tower to form an aqueous (meth)acrylic acid stream; (D) feeding the aqueous (meth)acrylic acid stream to a distillation column, wherein it is subjected to azeotropic distillation in the presence of at least one distillation solvent to form a (meth)acrylic acid solution substantially free of water.
In a fourth aspect of the present invention, there is provided a process for preparing (meth)acrylic acid, including the steps of (A) feeding to an absorption tower a mixed product gas from the catalytic oxidation of at least one hydrocarbon material with a molecular oxygen containing gas, and an aqueous stream comprising recycled wastewater and less than 3.0 percent by weight acetic acid, wherein prior to being added to the aqueous stream at least a portion of the recycled wastewater stream is stripped of undesirable components in a stripping column using a stripping gas which includes a waste gas stream; (B) contacting the mixed product gas with the aqueous stream in the absorption tower to form an aqueous (meth)acrylic acid stream; (C) stripping light ends from the aqueous (meth)acrylic acid stream; and (D) feeding the aqueous (meth)acrylic acid stream to a distillation column, wherein it is subjected to azeotropic distillation in the presence of at least one distillation solvent to form an (meth)acrylic acid solution substantially free of water.
FIG. 1 depicts a flow chart showing one embodiment of the process of the present invention.
FIG. 2 depicts a flow chart showing a second embodiment of the process of the present invention.
FIG. 3 depicts a flow chart showing a third embodiment of the process of the present invention.